1. Field of the Invention
This invention relates to devices for transmitting current to or from the body, and more particularly to the electrodes of these devices which are applied to the skin surface. Typical devices which use electrodes for electrotherapy include Transcutaneous Electrical Nerve Stimulator (TENS) devices, interferential current devices, and faradic current devices. Also, this invention applies to monitoring electrodes and electrosurgical return electrodes. Pertinent also are electrodes impregnated with chemical, i.e., electrodes for iontophoreats treatments. Typical devices which use electrodes for monitoring include EEG and EKG machines.
2. Description of Related Art
Transcutaneous Electrical Nerve Stimulator ("TENS") units are used to manage acute pain or chronic intractable pain. Other than the TENS device, the most commonly-used treatments for pain have been drugs and surgery. However, analgesic drugs often are ineffective or have undesirable side effects, including the potential for drug abuse and addiction. In addition, pain surgery has a record of complications and undesirable short term effects.
On the other hand, patients treated with the TENS device have been found to begin ambulation much earlier than previously possible with other treatment methods. In addition, TENS devices have been found to reduce the necessary dosages of analgesic drugs, when these two treatment methods are used together.
Other electrotherapy devices also provide physicians with improved or alternative methods for administering to patients.
Electrotherapy devices typically comprise a source of electrical current attached to an electrode through one or more conducting wires. In some electrotherapy devices, i.e., TENS devices, the electrical current may be pulsed and oscillatory. Monitoring devices typically comprise a system for detecting electrical current attached to a monitoring electrode through one or more conducting wires.
the electrical contact between skin and electrotherapy or monitoring electrodes is made by applying the electrode directly to the skin. In order to achieve electrical contact without injury or discomfort to the patient, electrodes generally comprise a means for current to flow into or out of the electrode, a means for distributing current evenly throughout the electrode, and a means for insulating the outer surface of the conducting elements. The means for current flow into or out of the electrode and the means for distributing the current throughout the electrode are typically imbedded in an organic medium with adhesive properties for skin. For example, electrotherapy electrode media such as agar gels and acrylate polymers have been employed with imbedded stainless steel mesh conducting wire in order to distribute the electrical current throughout the electrode media and thereby apply uniform electrical contact over a specific surface area of skin. The means for insulating the outer surface of the conducting elements typically includes an outer nonconducting layer that insulates the current in these elements.
Because problems may be encountered when attempting to adhere organic materials to water-based polymers, previous attempts to substitute aqueous conducting gels for nonconducting organic gels with imbedded steel mesh in electrodes have been unsuccessful due to the inability to bond the organic nonconducting insulator element to the aqueous gel component. Thus, previous attempts to bond the insulating layer to an aqueous conductive gel layer have produced poor attachments.
Additionally, many forces which tend to pull conducting wires out of electrotherapy or monitoring electrodes arise during the ordinary use of their various medical devices. For reasons outlined above, attempts to create an electrode using aqueous gels with sufficient adhesion to the insulating layer to retain the conducting wires during ordinary use of the device have been unsuccessful. Such previous electrodes have been able to withstand pulling forces of only two pounds or less before the electrode components may dissociate.
For reasons outlined above, previous self-adhering electrotherapy electrodes have employed nonaqueous gels. however, without an imbedded current distribution means as described above, nonaqueous gels fail to distribute current evenly throughout the gel, but instead transmit current to isolated "hot spots." In order to avoid this problem, previous electrodes, such as the Ceptor Stix TENS electrode, manufactured by Neuromedics, Inc. of Clute, Tex., have added a stainless steel lattice to distribute the current more evenly throughout the gel. This lattice had the additional function of maintaining the shape of the gel. however, the inclusion of the stainless steel lattice adds to the material and production costs of the electrode, thereby substantially increasing the cost of using the medical device for treatment of patients.
The need to discover a means for bonding the insulator layer to conductive aqueous gels of electrotherapy and monitoring electrodes resurfaced with the development of the aqueous gel products, Hydrogel and Stratum, by the Nepera Chemical Company, Inc., a subsidiary of Schering, A. G. of West Germany. Hydrogel, otherwise known as "irradiated polyox," is a smooth, uniform aqueous gel-filled polyethylene film comprising about 4% polyethylene and 96% water. Stratum, a form of Hydrogel with about 5% salt added, is an excellent conducting gel. Both Stratum and Hydrogel are adhesive to skin.
Stratum is capable of distributing electrotheraphy electrode current evenly throughout a medium up to two inches on a side which easily maintains its geometric form.
Previous attempts to use Stratum as an aqueous conducting gel in electrotherapy electrodes have not produced acceptable results due to the inability to bond the electrode insulating layer to the aqueous conducting gel layer. Although there has been much experimentation and discussion about how materials adhere to one another, the forces responsible for adhesion are not well understood. At one time, adhesion was believed to comprise a mechanical attachment wherein the liquid adhesive occupied cavities in the adherend where it hardened and was thus mechanically anchored somewhat below the surface of the adherend. This type of adhesion is commonly referred to as mechanical adhesion. Although mechanical adhesion may contribute a minor component to adhesive forces in some porous adherends, it is now generally considered that actual adhesive forces are due to primary and secondary valence interactions similar to those that hold the atoms and molecules of the adherents themselves together. This type of adhesion is referred to herein as chemical adhesion.
At one time, chemical adhesion was believed to arise when polar adhesives bonded to polar adherends or nonpolar adhesives bonded to nonpolar adherends, but that polar and nonpolar materials were not capable of bonding through adhesive forces. To the extent that many organic substances are substantially less polar than water, this hypothesis suggests that it may be difficult to adhere an organic insulating layer to an aqueous conducting gel of an electrotherapy electrode, except, perhaps, by means of mechanical adhesion. however, the hypothesis has little predictive value in practice since no adequate means existed for determining polarity. In fact, numerous exceptions were noted when dipole moments were taken as the measure of polarity.
Modern theories of the nature of adhesion require actual wetting of the adherend by the adhesive to achieve contact within molecular distances. Given close proximity, adhesion occurs if the interfacial boundary energy of the adhesive and the adherend is less than the sum of the surface energies of the adhesive and of the adherend. Empirical tests of this hypothesis are difficult to conduct because adequate means of measuring these energies for adhesives and adherends are usually lacking. however, a theoretical basis for the nature of adhesive forces based on general physical and chemical principles have been developed. The emerging view is that adhesive forces are a result of primary and secondary chemical bonds. For example, a review by F. W. Reinhart in J. Chem. Ed. 31 128 (March, 1954) suggests that primary bonds in adhesion include electrovalent, covalent, and coordinate covalent bonds involving either the transfer or sharing of electrons among the atoms and molecules of the adhesive and adherend. An example of an electrovalent bond that has been reported in adhesion is the strong bond between copper and sulfur in rubber compounds. An example of a covalent bond in adhesion may be the bond produces by treating glass with a chlorosilane. Coordinate covalent bonds are believed to be important in bonding organic adhesives to materials containing carboxyl or hydroxyl groups, such as cellulose.
Second chemical bonds are the result of van der Waals forces arising from residual energies. Van der Waals forces are stronger in molecules comprising different atoms than in atoms or molecules of the same atom. Van der Waals forces are also stronger in asymmetric molecules that have unequal electron distributions than they are in symmetrical molecules or nearly symmetrical molecules with smaller dipole moments. Van der Waals forces are thought to result from orientation forces of permanent electrical dipole molecules, from induction effects of permanent dipoles on polarizable molecules, and from dispersion forces due to internal electron motions independent of the dipole moments.
Internal stress tests the integrity of adhesive bonds. These internal stresses are the result of faults that may develop within the adhesive layer and vary with the adhesive composition, the bonding conditions, thermal and moisture changes in the adhesive film or in the adherends after bonding, and external loading during use, which in turn will be influenced substantially by the design and geometry of the adhering surface. Since these stresses reduce the theoretical strength of the adhesive bond, the composition of the adhesive and adherend components, the conditions of bonding, moisture content and temperature of the bonded unit, external forces acting on the bonded unit, and the design and geometry of the adhering surfaces are all important considerations in the invention of strongly adhesive components of electrotherapy and monitoring electrodes.
Additionally, electrodes using Stratum gel have a tendency to dry near the edges when the electrode is in use and exposed to air.